fix: allow using case-insensitive user-agent key

[olavloite]

Adding a custom user-agent was only possible by using an all lower-case key. A header key should however be case-insensitive. This PR changes the merging of a custom user-agent value with the default user-agent to use a case-insensitive comparison.

[codecov]

Codecov Report

Merging #1110 (ea7dea0) into master (37ca990) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@ Coverage Diff @@ ## master #1110 +/- ## ============================================ - Coverage 84.99% 84.99% -0.01%  Complexity 2727 2727 ============================================ Files 156 156 Lines 14410 14416 +6 Branches 1379 1381 +2 ============================================ + Hits 12248 12253 +5  - Misses 1593 1596 +3  + Partials 569 567 -2 

Impacted Files	Coverage Δ	Complexity Δ
...m/google/cloud/spanner/spi/v1/GapicSpannerRpc.java	80.37% <100.00%> (+0.14%)	91.00 <0.00> (+2.00)
...m/google/cloud/spanner/connection/SpannerPool.java	87.16% <0.00%> (-0.54%)	33.00% <0.00%> (ø%)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.01% <0.00%> (ø)	74.00% <0.00%> (ø%)
...a/com/google/cloud/spanner/SessionPoolOptions.java	69.53% <0.00%> (ø)	17.00% <0.00%> (-1.00%)

---

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 37ca990...ea7dea0. Read the comment docs.

[thiagotnunes]

Thanks for the fix @olavloite