Relax validation of HTTP header values to retain L/M behavior

This relaxes some validation logic newly introduced in the version
of OkHttp used in N.

This change leaves the character code validation stricter
than it was in M by still preventing control codes like \n, \r,
backspace and delete. It does allow developers to pass
Java characters > 7F to addRequestProperty() and also receive
headers from servers which contain characters > 7F.

Android's HttpURLConnection does not follow the HTTP spec as
it encodes request header values and interprets response
headers as UTF-8 and not ISO-8859-1.

If a server is expecting or sending ISO-8859-1 encoded characters
>7F in headers then these will still be corrupted or misinterpreted
by Android. However, this has been the behavior since L and is not
changed here.

The OkHttp change which caused characters >7F to generate an
IllegalArgumentException and partially reverted here:
https://github.com/square/okhttp/pull/1785

See also: https://github.com/square/okhttp/issues/1998
https://github.com/square/okhttp/issues/2016
for recent upstream bugs.

Bug: 28867041
Bug: https://code.google.com/p/android/issues/detail?id=210205
Change-Id: Ibdf14d819411a12fcc78d012bfca97db048b7e6e