Fix svg writer for StringIO objects

[dmcdougall]

Fix #1373.

This seems like a really simple fix. I'm a little concerned that it may now be possible to pass something to print_svg that doesn't make sense. Hopefully someone will correct me if this is the case.

Also, I kind of feel like the write should be in a try/finally block, like print_png in the agg backend.

[dmcdougall]

Also, ByteIO and StringIO both have close methods that aren't being called after the figure has been written. Perhaps I should fix this too...

[mdboom]

This seems to have created a new Travis failure about seeking on a closed file. Needs further investigation.

[mdboom]

Regardless of the closing, I can confirm that the initial fix does the trick. Thanks.

[dmcdougall]

I assumed one would not seek the file once the figure has been written. Maybe I shouldn't assume this. Investigating...

[dmcdougall]

I see the problem. The test re-reads the saved figure to test that it's a valid svg file. There are two options:

1. Don't close the user's file descriptor. To implement this I will undo the change that closes the file descriptor and the test will pass.

2. Assume the user wants the file descriptor closed after writing. Possibly not the correct assumption if given a specially constructed BytesIO or StringIO object. To fix this, the test will need changing to re-open the file and check its contents. The test will pass.

I think I should probably undo the close change and assume the user doesn't want specially constructed file descriptors closed. Thoughts?

[dmcdougall]

Ok, I'm making a decision. This PR should fix the original problem and not try to fiddle with other things. I will undo the close change and keep the original solution.

[mdboom]

I think that's right, having just looked at this again. I think the intent is that only things that are created within that function should be closed.

[dmcdougall]

@mdboom Yeah. I think I'd be annoyed if functions started freeing memory when I didn't expect it. The tests pass now, too.

[mdboom]

Great! Merging.