Prevent #parse_buffer from altering input buffer

[HosokawaR]

Fixes #453
This PR prevent #parse_buffer from altering input buffer.

As stated in the issue, it is more intuitive for parse_buffer to not change the input buffer.
However, the internal call Zip::File.open_buffer changes the input buffer if called with block.
Therefore, I changed the calling method so that it does not use block.

[weshatheleopard]

You see, I'm temped to NOT do anything about this issue in RubyXL since all that it does is passing the buffer to RubyZip; from that point, it's RubyZip responsebility: if it modifies buffer, it's it's fault, not RubyXLs. I do not appreciate this behavior, as there's no reason why it should ever modify the buffer, but that's what they do.
I figured out that when I pass a String to RubyZip, then .freeze'ing it before passing it over does the trick.
I don't have a problem adding a warning about that to the documentation. But I think RubyZip's issues need to be handled with its developers.

[HosokawaR]

Thank you for your reply.

it's RubyZip responsebility:

I agree. This PR will be closed for now.