Set Context#initialize instance variables before squashing assigns

[dylanahsmith]

Problem

While rebasing a liquid-c PR (Shopify/liquid-c#60), I noticed in this test test failure

ContextTest#test_context_initialization_with_a_proc_in_environment:
NoMethodError: undefined method `each' for nil:NilClass
    /Users/dylants/src/liquid/lib/liquid/strainer_factory.rb:27:in `strainer_from_cache'
    /Users/dylants/src/liquid/lib/liquid/strainer_factory.rb:14:in `create'
    /Users/dylants/src/liquid/lib/liquid/context.rb:56:in `strainer'
    /Users/dylants/src/liquid/lib/liquid/context.rb:170:in `c_evaluate'
    /Users/dylants/src/liquid/lib/liquid/context.rb:170:in `[]'
    ../liquid/test/integration/context_test.rb:470:in `block in test_context_initialization_with_a_proc_in_environment'
    /Users/dylants/src/liquid/lib/liquid/context.rb:207:in `lookup_and_evaluate'
    /Users/dylants/src/liquid/lib/liquid/context.rb:271:in `block (2 levels) in squash_instance_assigns_with_environments'
    /Users/dylants/src/liquid/lib/liquid/context.rb:269:in `each'
    /Users/dylants/src/liquid/lib/liquid/context.rb:269:in `block in squash_instance_assigns_with_environments'
    /Users/dylants/src/liquid/lib/liquid/context.rb:268:in `each_key'
    /Users/dylants/src/liquid/lib/liquid/context.rb:268:in `squash_instance_assigns_with_environments'
    /Users/dylants/src/liquid/lib/liquid/context.rb:37:in `initialize'
    ../liquid/test/integration/context_test.rb:470:in `new'
    ../liquid/test/integration/context_test.rb:470:in `test_context_initialization_with_a_proc_in_environment'

that in Liquid::Context#initialize we call squash_instance_assigns_with_environments, which can evaluate Procs in the environment, which means that we are passing in a partially initialized Context object into those Proc objects.

Solution

Move the call to squash_instance_assigns_with_environments to the end of Liquid::Context#initialize so that we aren't exposing it in a partially initialized state.