fixing the localfs reader writer truncate bug #52

[epsniff]

fixes #52

The localfs store wasn't correctly truncating the file when a new writer was created.

[epsniff]

My new testcases exposed a bug in SFTP so I'll fix that one tomorrow, in this pr.

[codecov-io]

Codecov Report

Merging #53 into master will decrease coverage by 0.12%.
The diff coverage is 37.5%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
- Coverage   65.02%   64.89%   -0.13%     
==========================================
  Files          13       13              
  Lines        1990     1997       +7     
==========================================
+ Hits         1294     1296       +2     
- Misses        488      491       +3     
- Partials      208      210       +2

Impacted Files	Coverage Δ
sftp/store.go	61.66% <25%> (-0.35%)	⬇️
localfs/store.go	69.37% <50%> (-0.41%)	⬇️

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[araddon]

looks good, thank you!

[mh-cbon]

Hi guys,

i wonder why we d try to delete a file that does not exist.

	if !m.Exists(name) {
+		if err := m.Delete(ctx, name); err != nil {
+			gou.Errorf("failed to delete existing file %v %v", name, err)
+			return nil, err
+		}
+	}

Besides, i think this line https://github.com/lytics/cloudstorage/blob/master/sftp/store.go#L251
should look for an error such as : &errors.errorString{s:"file does not exist"}