Improve performance of xml character filtering

[jamespeacock]

Regexp can filter out invalid bytes from XML strings faster than join()

[codecov[bot]]

Codecov Report

Merging #257 into master will increase coverage by 0.08%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #257      +/-   ##
=========================================
+ Coverage   94.31%   94.4%   +0.08%     
=========================================
  Files           7       7              
  Lines        1285    1286       +1     
=========================================
+ Hits         1212    1214       +2     
+ Misses         73      72       -1

Impacted Files	Coverage Δ
pysolr.py	89.71% <100%> (+0.15%)	:arrow_up:
run-tests.py	90% <0%> (ø)	:arrow_up:
tests/test_client.py	99.61% <0%> (ø)	:arrow_up:

---

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 ee28b39...8b3c062. Read the comment docs.

[acdha]

We should probably get some before/after benchmarks for this, too.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[cclauss]

Please rebase.