Mail::Field responds_to? delegated methods properly

Mail::Field delegates method calls to the contained field using `method_missing` but did not also override `respond_to?`. As an example, attempts to call `mail[:to].try(:decoded)` always return `nil` instead of the decoded field value.
`respond_to?` used instead of `respond_to_missing?` for Ruby 1.8.7 compatibility. (See http://robots.thoughtbot.com/always-define-respond-to-missing-when-overriding)
Tests pass on MRI 1.8.7-p375, 1.9.3-p547, 2.0.0-p247.