StreamIndenter fix for blank lines

[jsiirola]

Fixes: N/A

Summary/Motivation:

This updates the StreamIndenter to rstrip the prefix when outputting blank lines. This prevents outputting spaces on otherwise blank lines.

Changes proposed in this PR:

• rstrip the prefix when writing otherwise blank lines

Legal Acknowledgement

By contributing to this software project, I agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[coveralls]

Coverage decreased (-0.007%) to 62.239% when pulling 5d08b09 on indenter-fix into a210d03 on master.

[coveralls]

Coverage increased (+0.04%) to 62.285% when pulling d299060 on indenter-fix into a210d03 on master.

[codecov-io]

Codecov Report

Merging #58 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   63.09%   63.13%   +0.03%     
==========================================
  Files          87       87              
  Lines        8693     8702       +9     
==========================================
+ Hits         5485     5494       +9     
  Misses       3208     3208

Impacted Files	Coverage Δ
pyutilib/misc/indent_io.py	100% <100%> (ø)	⬆️

---

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 a210d03...d299060. Read the comment docs.