teach #inspect how to build string without appending to a frozen string

[technoweenie]

The Rubocop effort included (#868) taking advantage of an optimization that makes all string literals immutable. However, it turns out we don't test #inspect on the Faraday::Options types.

You can trigger this bug easily with the current master branch:

irb(main):003:0> Faraday.get "http://example.com"
Traceback (most recent call last):
        5: from /Users/rick/.rbenv/versions/2.5.3/bin/irb:11:in `<main>'
        4: from /Users/rick/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/faraday-1.0.0.pre.rc1/lib/faraday/options/env.rb:151:in `inspect'
        3: from /Users/rick/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/faraday-1.0.0.pre.rc1/lib/faraday/options/env.rb:151:in `each'
        2: from /Users/rick/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/faraday-1.0.0.pre.rc1/lib/faraday/options/env.rb:153:in `block in inspect'
        1: from /Users/rick/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/faraday-1.0.0.pre.rc1/lib/faraday/options.rb:154:in `inspect'
FrozenError (can't modify frozen String)