[CORS] Preflight algorithm and content types

Per the current CORS spec draft, cross-origin request with preflight  
algorithm is followed if Content-Type is set, and it is not one of the  
three predefined types. However, the preflight algorithm itself  
immediately decides to skip preflight if these conditions are true:

  * For request method there either is a method cache match or it is a  
simple method.
  * For every header field name of custom request headers there either  
is a header cache match or it is a simple header.

It seems that a check for content type should be added here, as well.

- WBR, Alexey Proskuryakov