actually do not camel case fx prefs

[glaszig]

Description

firefox preferences must not be camelized. this was intended in eb2d017 but failed simply due to a type mismatch.

Motivation and Context

i wanted to set intl.accepted_languages but in a firefox instance it ended up as intl.acceptedLanguages and thus having no effect.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

all tests in the scope of my changes are passing. many others don't. i consider them being wip.

before this pr:

bundle exec rspec spec/unit/selenium/webdriver/firefox/options_spec.rb:153
Run options: include {:locations=>{"./spec/unit/selenium/webdriver/firefox/options_spec.rb"=>[153]}}
F

Failures:

  1) Selenium::WebDriver::Firefox::Options#add_preference does not camelize preferences
     Failure/Error: expect(prefs).to eq(expected)

       expected: {"intl.accepted_languages"=>"en-US"}
            got: {"intl.acceptedLanguages"=>"en-US"}

       (compared using ==)

       Diff:
       @@ -1 +1 @@
       -"intl.accepted_languages" => "en-US",
       +"intl.acceptedLanguages" => "en-US",

     # ./spec/unit/selenium/webdriver/firefox/options_spec.rb:158:in `block (3 levels) in <module:Firefox>'

Finished in 0.01195 seconds (files took 0.40692 seconds to load)
1 example, 1 failure

Failed examples:

rspec ./spec/unit/selenium/webdriver/firefox/options_spec.rb:153 # Selenium::WebDriver::Firefox::Options#add_preference does not camelize preferences

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[titusfortner]

This is what happens when I don't write a test. Thanks for your submission.