Don't use CharsetReader when Part is attachement

[XiaoMouR]

For this issue

[coveralls]

Coverage remained the same at 85.82% when pulling 6dac6f5 on leibiao:patch-1 into b6ea523 on jhillyerd:master.

[jhillyerd]

This is a really nice find. Thank you! I was really dreading digging into bug #34 but I have a feeling it has the same root cause as #42 you filed.

I've been very busy at work and it looks like I've let a back log of PRs pile up, will try and spend some time this weekend getting them merged.

Would it be possible for you to add a corresponding test to part_test.go? You could create a new testdata/parts file based on https://github.com/leibiao/enmime/blob/master/testdata/parts/multibase64.raw . Please try to limit the size of the test base64, I don't want to bloat the repository unnecessarily.