Refactor to add Rack env builder

[elct9620]

The Rack env build method is too large that hard to extend new behaviors. Add Hanami::Lambda::Env to improve it.

hanami-lambda/lib/hanami/lambda/rack.rb

Lines 43 to 57 in 8592279

	def build_env(event, headers, context)
	{
	::Rack::REQUEST_METHOD => event["httpMethod"],
	::Rack::PATH_INFO => event["path"] || "",
	::Rack::VERSION => ::Rack::VERSION,
	::Rack::RACK_INPUT => StringIO.new(event["body"] || ""),
	::Hanami::Lambda::LAMBDA_EVENT => event,
	::Hanami::Lambda::LAMBDA_CONTEXT => context
	}.tap do |env|
	content_type = headers.delete("Content-Type") ||
	headers.delete("content-type") ||
	headers.delete("CONTENT_TYPE")
	env["CONTENT_TYPE"] = content_type if content_type
	end.merge(headers.transform_keys { |k| "HTTP_#{k.upcase.tr('-', '_')}" })
	end