+ httpx: change default StringMarshaller to prefer UTF-8 encoding over ISO-8859-1, closes #498

So far the built-in StringMarshaller always used the ISO-8859-1 charset (the HTTP default charset). However, nowadays UTF-8 is the better choice. With this patch the StringMarshaller now renders w/ UTF-8 if the client accepts it, otherwise it will use whatever charset the client prefers.

 This change happens to require changes in quite a bit of tests that implicitly relied on the previous behavior.
 Note that the default `HttpEntity(String)` constructor now also uses UTF-8 rather than ISO-8859-1 as before.
Merge pull request #499 from spray/ticket/498

+ httpx: change default StringMarshaller to prefer UTF-8 encoding