Add support for http-path

[MarcoPolo]

Context here: multiformats/multiaddr#164

[sukunrt]

Can we add a few examples to the fuzzer here:

go-multiaddr/multiaddr_test.go

Line 107 in e0315b9

var good = []string{

[MarcoPolo]

good call on the fuzzing. I found a surprising behavior where if a variable sized component has a zero-len value it will not put anything into the multiaddr string. This means that the string /http-path/http is ambigous since it's not clear if it's http-path with no value and an http component, or an http-path with the value "http".

To disambiguate, I've made it a requirement for there to be at least some value in the http-path component. Which seems to be the same thing that other length prefixed sized components do (e.g. dns, p2p). This library should probably make it possible to represent components with a value that has len 0 (e.g. /http-path//http), but nothing currently needs this, so likely not a priority.