#33 Setting mtime on ZipEntry raises Exception on commit

[Ingo Tomahogh]

(Related: #1543984)
In version 0.9.4, setting the mtime of a ZipEntry like this:
zipentry.time = File.stat(zipentry.to_s).mtime
raises an Exception when Zip::ZipFile#commit() is called:
zip/zip.rb:1710:in `pack': can't convert nil into Integer (TypeError)

This is due to various errors in class Zip::ZipExtraField::UniversalTime:
@flag should be initialized to 0, not nil, or alternatively, @flag should be accessed as @flag.to_i (or not be used at all, see below).
The attribute writer methods should set the appriopriate bit in @flag:

def mtime=(value)
@mtime = value
@flag = @flag.to_i | 1
end

As I understand it, @flag is actually redundant, indicating which time values are really set. This is easy to detect in the ruby data, as @mtime etc. are initialized to nil. In #merge(), @flag is already ignored, anyway.

So you should probably better not use @flag at all, but generate the value on-the-fly according to the time values:

def flag
f = 0
@mtime and f |= 1
@atime and f |= 2
@ctime and f |= 4
f
end