Add tests for the keyFilter feature & apply filter everywhere

[cweinberger]

CHANGELOG:

• NEW: Headers are filtered out based on keyFilters
• NEW: URL query params are filtered out based on keyFilters
• NEW: Added tests for keyFilters feature

NOTE:

• This PR does not change any API contracts
• It changes the behaviour though

DISCUSSION:

• We might add the ability to filter non-json-bodies as well (e.g. www-form-urlencoded)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@98c6a17). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #62   +/-   ##
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  Coverage          ?   87.74%           
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  Files             ?        8           
  Lines             ?      204           
  Branches          ?        0           
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  Hits              ?      179           
  Misses            ?       25           
  Partials          ?        0

Impacted Files	Coverage Δ
Sources/Bugsnag/BugsnagConfig.swift	100% <100%> (ø)
Sources/Bugsnag/Bugsnag.swift	97.91% <100%> (ø)

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[siemensikkema]

Looks good! Maybe one optimization could be to create a Set from filters and passing that into the different filtering steps. Doing .contains should be faster but maybe not worth it unless we expect large amounts of filters.

[cweinberger]

Added the suggested change and also a brief introduction in the README.md