Use faster === for regexp matching

We do not rely on the result of the matching, or any part of it, just
whether it matches or not, so === is preferable as it's slightly faster
by not creating extra matching objects.

Also remove the condition for nil as it responds to === and always
returns false.

    >> nil === 'zomg'
    => false

lib/rollbar/scrubbers/params.rb

@@ -53,7 +53,7 @@ def scrub(params, options)
  return scrub_array(params, options) if params.is_a?(Array)
 
  params.to_hash.inject({}) do |result, (key, value)|
- if fields_regex && fields_regex =~ Rollbar::Encoding.encode(key).to_s
+ if fields_regex === Rollbar::Encoding.encode(key).to_s
  result[key] = scrub_value(value)
  elsif value.is_a?(Hash)
  result[key] = scrub(value, options)