Fix issue with opting out of headers when using header_hash method

[oreoshake]

The header_hash opt-out code was not consistent with the non-header-hash code. Mainly opting out of a header meant you would get an exception:

     NoMethodError:
       undefined method `name' for nil:NilClass
     # ./lib/secure_headers.rb:63:in `block in header_hash'
     # ./lib/secure_headers.rb:54:in `each'
     # ./lib/secure_headers.rb:54:in `inject'
     # ./lib/secure_headers.rb:54:in `header_hash'

I'm going to see if this issue exists on the 3.x branch as well.

/cc @moratorius