Using go1.6

Some combination of fields tar.Header are simply impossible to encode for any format. Some example situations that we currently "succeed" in, but should really error on:

• Example: String fields can incorrectly have the NUL character in them. This is not allowed since the tar format uses C-strings.
• Example: PAX attribute keys incorrectly allow the '=' character. This is not allowed since PAX uses the 'length keyword=value' format to encode attributes.
• Example: Encoding of overflow integers causes the NUL terminator to disappear. In this situation, we should upgrade to using the base256 format, rather overflowing the base8 format.
• Example: Encoding of negative integers is incorrectly allowed in octal format. The GNU format seems to handle negative numbers, but only in the base256 format, not base8.