Support field hash_key: values that are not valid ruby method names

[dpkp]

ruby-2.5.1 / graphql-1.8.9

The schema default_resolver crashes if a field uses a non-standard hash_key, like so:

field :foo, String, hash_key: :"foo/bar"
field :bar, String, hash_key: "2nd test case"

SyntaxError (/usr/local/rvm/gems/ruby-2.5.1/gems/graphql-1.8.9/lib/graphql/schema/member/has_fields.rb:121: syntax error, unexpected '/', expecting ';' or '\n'
            def foo/bar(**args)
                   ^
/usr/local/rvm/gems/ruby-2.5.1/gems/graphql-1.8.9/lib/graphql/schema/member/has_fields.rb:124: syntax error, unexpected keyword_end, expecting end-of-input
            end
            ^~~):

Putting aside the wisdom of such hash_keys, this PR is intended to allow the user to use any valid string or symbol as a hash_key. I've updated the spec tests to cover.