1.x: Ensure only ASCII characters are used in the User-Agent header.

The charset for HTTP headers has to be ASCII at best
and is ill defined at worst.

My Application has a Bundle Name with Umlauts in it
which results in messy Webserver log files due to the
"strange" engoding of the App name.
See http://filez.foxel.org/image/0c3a2n0N1H2Y for example

To me it seems, that AFNetworking is meant to transcode the
App name to ASCII but the implementation is lacking and thus
only encodes to Latin1/ISO8859-1.
This patch - although not tested with Asian scripts - should
result in User-Agent strings always beeing ASCII while remaining
mostly unchanged.

So `Überland/0.16.27` becomes `Uberland/0.16.27` which is much
more standards compliant.