8998: Monkey patch URI.decode_www_form_component to validate efficiently.

8998: Monkey patch URI.decode_www_form_component to validate efficiently.

Rack uses the standard library method URI.decode_www_form_component to process
parameters.  This method first validates the string with a regular expression,
and then decodes it using another regular expression.  Ruby 2.1 and earlier has
a bug is in the validation; the regular expression that is used generates many
backtracking points, which results in exponential memory growth when matching
large strings.  The fix is to monkey-patch the version of the method from Ruby
2.2 which checks that the string is not invalid instead of checking it is
valid.