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%)

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[thiagotnunes]

Thanks for the fix @olavloite