assume "unknown" platform if _exec_command throws exception

[mrk-its]

It seems that it solves problem of forbidden execution of external commands on some servers - after setting platform to "unknown" self.exec_command is never called again.

Fixes #34

[codecov]

Codecov Report

Merging #35 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
+ Coverage   96.94%   96.96%   +0.02%     
==========================================
  Files           5        5              
  Lines         327      330       +3     
==========================================
+ Hits          317      320       +3     
  Misses         10       10

Impacted Files	Coverage Δ
fs/sshfs/sshfs.py	95.63% <100%> (+0.05%)	⬆️

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[mrk-its]

cc: @althonos please let me know what do you think about that. Thanks!

[althonos]

@mrk-its : thanks for the patch! there's an issue with Travis-CI (which is not shown in the PR checks since recently the Github/Travis-CI integration started to fuck up): Mock.assert_called was only introduced in Python 3.6, so the test fails on 3.5.

Simply wrap it that way (there's no need to refactor to handle the test in 3.5, let's just skip it) and we should be clear to merge:

    def test_exec_command_exception(self):
        with utils.mock.patch.object(
            self.fs.delegate_fs(),
            '_exec_command',
            side_effect=paramiko.ssh_exception.SSHException()
        ) as _exec_command:
            self.assertEquals(self.fs.delegate_fs().platform, "unknown")
            if sys.version_info[:2] != (3,5):
                _exec_command.assert_called()

[mrk-its]

@althonos done!

[althonos]

Thanks again ! I'll merge and push a patch release shortly.