Fix compilation of input_adapter(container) in edge cases

[jasujm]

This fixes a compilation issue with the library if trying to use containers that don't have non-member begin() and end() functions via ADL.

Example of a test program that fails with the current library version:

$ g++ --version
g++ (Ubuntu 10.2.0-13ubuntu1) 10.2.0
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cat test.cc
#include <iostream>

#include <nlohmann/json.hpp>

const char DATA[] = R"("Hello, world!")";

struct MyContainer {
    const char* begin() const { return DATA; }
    const char* end() const { return DATA + sizeof(DATA); }
};

int main()
{
    MyContainer c;
    std::cout << nlohmann::json::parse(c) << "\n";
    return 0;
}
$ g++ test.cc
In file included from test.cc:3:
/usr/include/nlohmann/json.hpp: In instantiation of ‘static nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType> nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::parse(InputType&&, nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::parser_callback_t, bool, bool) [with InputType = MyContainer&; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long int; NumberUnsignedType = long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>; nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::parser_callback_t = std::function<bool(int, nlohmann::detail::parse_event_t, nlohmann::basic_json<>&)>]’:
test.cc:15:41:   required from here
/usr/include/nlohmann/json.hpp:6654:37: error: no matching function for call to ‘input_adapter(MyContainer&)’
etc...

This PR fixes the issue with types that only have member begin() and end() functions. Please see the commit for further details.

---

Pull request checklist

Read the Contribution Guidelines for detailed information.

• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header file single_include/nlohmann/json.hpp. The whole process is described here.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Specifically, I am aware of compilation problems with Microsoft Visual Studio (there even is an issue label for these kind of bugs). I understand that even in 2016, complete C++11 support isn't there yet. But please also understand that I do not want to drop features or uglify the code just to make Microsoft's sub-standard compiler happy. The past has shown that there are ways to express the functionality such that the code compiles with the most recent MSVC - unfortunately, this is not the main objective of the project.
• Please refrain from proposing changes that would break JSON conformance. If you propose a conformant extension of JSON to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[coveralls]

Coverage remained the same at 100.0% when pulling 467f622 on jasujm:jasujm/input_adapter_custom_container into dfedefb on nlohmann:develop.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!