This is the recommended approach (since NetUtil can still do some main-thread
I/O for files) and avoids warnings in the console.
For getContentsAsync(), also sends nsIURIs and string URIs to
Zotero.HTTP.request(), which should be used instead.
This makes getBinaryContentsAsync() much slower (due to the conversion from an
array of bytes to a binary string), but it's only used in tests. For one test
that compares two large files, use MD5 instead.
Restore prepopulated charset table, but this time with just the
encodings from the WHATWG Encoding Standard. Assigning a charset to
Zotero.Item::attachmentCharset runs the value through
Zotero.CharacterSets.toCanonical() automatically.
This migrates attachment charsets to the new canonical values, clearing any
that are unsupported.
Other legacy mappings could still be added back, as disussed in #760.
