Add base64 to gemspec #498

Job	Run time
test-opensearch (2.5)	1m 59s
test-opensearch (2.6)	2m 8s
test-opensearch-faraday-1 (2.6)	1m 59s
test-opensearch (2.7)	2m 22s
test-opensearch-faraday-1 (2.7)	2m 19s
test-opensearch (3.0)	2m 12s
test-opensearch-faraday-1 (3.0)	1m 44s
test-opensearch (3.1)	2m 16s
test-opensearch-faraday-1 (3.1)	1m 48s
test-opensearch (3.2)	1m 57s
test-opensearch-faraday-1 (3.2)	2m 9s
test-opensearch (jruby-9.3)	3m 24s
test-opensearch-faraday-1 (jruby-9.3)	2m 4s
test-opensearch-security (2.5)	1m 56s
test-opensearch-security (2.6)	2m 9s
test-opensearch-security (2.7)	2m 46s
test-opensearch-security (3.0)	2m 29s
test-opensearch-security (3.1)	1m 56s
test-opensearch-security (3.2)	1m 41s
test-opensearch-security (jruby-9.3)	1m 29s
	42m 47s

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