Add dependency on base64 gem.

Author: ioquatix

async-http.gemspec

@@ -22,6 +22,7 @@ Gem::Specification.new do |spec|
 	spec.add_dependency "async", ">= 1.25"
 	spec.add_dependency "async-io", ">= 1.28"
 	spec.add_dependency "async-pool", ">= 0.2"
+	spec.add_dependency "base64"
 	spec.add_dependency "protocol-http", "~> 0.25.0"
 	spec.add_dependency "protocol-http1", "~> 0.17.0"
 	spec.add_dependency "protocol-http2", "~> 0.15.0"