[#539] Use non-destructive gsub, fix test stubbing

[rsullivan00]

Closes #539

Apparently I like breaking stuff with my changes 😭 . This PR fixes gsub! from breaking on frozen string literals.

Some of the tests around this were actually stubbing the body call directly, instead of setting the data. I've changed these tests to inject the data instead. As a high-level note, a lot of tests for Parser are stubbing private methods, which I don't think is a good way to structure tests.

We should be able to test all use cases of Parser through the external API. When we stub, or test directly, internal methods, any implementation change can break any number of other tests, even if the change doesn't affect external behavior.

[jnunemaker]

As a high-level note, a lot of tests for Parser are stubbing private methods, which I don't think is a good way to structure tests.

Agreed. I wasn't the maintainer when most of the parsing stuff went in (took some time away) and just haven't had the energy to clean things up. Thanks!

[henrik]

I guess this is the fix for this issue?

07:45:09 >> HTTParty.get("https://auctionet.com/api/v2/companies.json").parsed_response
Encoding::CompatibilityError: incompatible encoding regexp match (UTF-8 regexp with ASCII-8BIT string)
	from /Users/henrik/.asdf/installs/ruby/2.4.1/lib/ruby/gems/2.4.0/gems/httparty-0.15.2/lib/httparty/parser.rb:120:in `gsub'

EDIT: Yes, it is.

Thanks for fixing it! Was just about to report it.