Add containsHeader to Http.RequestHeader.

It's not clear that `Http.RequestHeader.getHeader` operates in a case-insensitive way.  This commit clarifies the operation in the method's Javadoc.  Because `getHeader` is case-insensitive the developer is forced to do:

    getHeader("myHeaderName") != null

to determine if the Request contains the header, and he may make the mistake of doing

    headers().containsKey("myHeaderName")

which is case-sensitive.  This commit adds a convenience method `containsHeader` which is case-insensitive.