R1:d2cff6a2cf01

Transcode the HTML part of incoming email into UTF-8 as well

Summary:
D1093 did this for just the text/plain part of incoming
email. Most text/html parts choose to either use entity encoding
//or// are already UTF-8, thus obviating the need to transcode the
HTML part. However, this is not always the case, and leads to dropped
messages, by way of:

```
EXCEPTION: (Exception) Failed to JSON encode value (#5: Malformed UTF-8 characters, possibly incorrectly encoded): Dictionary value at key "html" is not…