Don't delete the whole destination folder while saving entityset

[chidauri]

Pull Request Description

Fixes #706 as the title suggests. The new functionality is to replace the conflicting files only and not the whole path.

---

After creating the pull request: in order to pass the changelog_updated check you will need to update the "Future Release" section of docs/source/changelog.rst to include this pull request.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #717 into master will decrease coverage by 0.02%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master     #717      +/-   ##
==========================================
- Coverage   97.66%   97.64%   -0.03%     
==========================================
  Files         118      118              
  Lines       10289    10301      +12     
==========================================
+ Hits        10049    10058       +9     
- Misses        240      243       +3

Impacted Files	Coverage Δ
...etools/tests/entityset_tests/test_serialization.py	98.08% <100%> (+0.09%)	⬆️
featuretools/entityset/serialize.py	95.12% <50%> (-3.63%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5b6873c...c738a87. Read the comment docs.

[rwedge]

Thanks again, @chidauri. Could you add a test case confirming that this no longer deletes unrelated files in the destination folder?

[rwedge]

this test name could be updated to provide more context about that it is testing extra files are not removed

[chidauri]

Is it okay now?

[rwedge]

Looks like there's an issue with py35 / py27 tests.

[chidauri]

Fixed :)

[rwedge]

@chidauri thanks for adding the test case! I think it would also be good to test that old serialization files (e.g. data_description.json) get overwritten.

[rwedge]

Looks good!