Added possibility to skip charset from header in UrlEncodedFormatter

[egorrishe]

Q	A
Is bugfix?	no
New feature?	yes
Breaks BC?	no
Tests pass?	yes

Implementing API integration, I faced with case when API works properly only with header:
Content-Type: application/x-www-form-urlencoded
But default header (with charset) works unexpectedly. I.e. :
Content-Type: application/x-www-form-urlencoded; charset=UTF-8

So I suggest to add possibility to skip charset.

Of course, it's possible to achieve the same goal and without this PR.
In my project I've override UrlEncodedFormatter with custom class and used it.

But such possibility from the box is much clearer.
Besides that it's really tiny PR.

[samdark]

Thanks.