Need a way to specify ContentType in ViewResult \ PartialViewResult

[pranavkm]

ContentResult has a way to specify the type and encoding for the result, but we're missing these properties on ViewResult \ PartialViewResult. It's currently hardcoded to send `text/html; charset=utf8'.

[pranavkm]

Related work item: While making this change, we need to ensure that we do not write the preamble \ BOM when a user specifies the Encoding. This is to keep parity with ContentEncoding in System.Web. The StreamWriter that we use to write to the Response writes the preamble by default.

[kichalla]

commit 7e62325