Reloading Rakefile causes already initialized constant errors

[dankohn]

In this test, I'm reloading the Rakefile and in order to rerun the fixtures. This causes the following spurious errors:

$ rake test
/Users/dan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/secure_headers-3.3.1/lib/tasks/tasks.rake:1: warning: already initialized constant INLINE_SCRIPT_REGEX
/Users/dan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/secure_headers-3.3.1/lib/tasks/tasks.rake:1: warning: previous definition of INLINE_SCRIPT_REGEX was here
/Users/dan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/secure_headers-3.3.1/lib/tasks/tasks.rake:2: warning: already initialized constant INLINE_STYLE_REGEX
/Users/dan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/secure_headers-3.3.1/lib/tasks/tasks.rake:2: warning: previous definition of INLINE_STYLE_REGEX was here
/Users/dan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/secure_headers-3.3.1/lib/tasks/tasks.rake:3: warning: already initialized constant INLINE_HASH_SCRIPT_HELPER_REGEX
/Users/dan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/secure_headers-3.3.1/lib/tasks/tasks.rake:3: warning: previous definition of INLINE_HASH_SCRIPT_HELPER_REGEX was here
/Users/dan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/secure_headers-3.3.1/lib/tasks/tasks.rake:4: warning: already initialized constant INLINE_HASH_STYLE_HELPER_REGEX
/Users/dan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/secure_headers-3.3.1/lib/tasks/tasks.rake:4: warning: previous definition of INLINE_HASH_STYLE_HELPER_REGEX was here

I'm submitting a PR for the simple fix momentarily.
Cc @david-a-wheeler