Tiny bug (fix) in entity_tmail.rb's from_tmail(tmail)

Seems there are instance variables in this method where there shouldn't be. Replace each so the method looks like this and you won't get any whiny nil errors about nil.header calls:

```
def from_tmail(tmail)
raise ArgumentError, "Expecting a TMail::Mail object." unless tmail.is_a?(TMail::Mail)
@headers ||= Hash.new {|h,k| tmail.header[k].to_s }
if multipart?
@content = tmail.parts.collect { |tpart| Entity.new.from_tmail(tpart) }
else
set_content tmail.body # TMail has already decoded it, but we need it still encoded
end
end
```

Inspect entity_tmail.rb's from_tmail(tmail) method and compare its instance-variable usage with the implementation shown in the issue. Run the existing test suite if available. Done means the method no longer produces nil.header errors and matches the requested variable scope.

Written by the indexing model from the issue text.