Fix Request#accept? NoMethodError

Allow Request#accept? to return false when type parameter does not
match any of the preferred types. When `preferred_type(type)` returns
nil, `include?` will throw a NoMethodError. By converting output to a
string, `include?` will return false.
Fix incorrect test title

This commit fixes an incorrect test title that misinformed what the
test was meant to prove.
Merge branch 'master' of https://github.com/sinatra/sinatra into fix-Request#accept-no_method_error

* 'master' of https://github.com/sinatra/sinatra:
  fixes the build by disabling path traversal protection for this test
Make Request#accept? test name more descriptive

This commit modifies the Request#accept? test by making it more verbose
and descriptive of the scenario in which it validates.
Merge pull request #805 from sbonami/fix-Request#accept-no_method_error

Fix Request#accept? NoMethodError