Restore compatibility with Ruby 1.8.7

The Base64 module does not have a strict_encode64 method in Ruby 1.8.

lib/twitter/client.rb

@@ -20,7 +20,6 @@
 require 'twitter/error/client_error'
 require 'twitter/error/decode_error'
 require 'simple_oauth'
-require 'base64'
 require 'uri'
 
 module Twitter
@@ -124,10 +123,14 @@ def connection
     #
     # @return [String]
     def bearer_token_credentials_auth_header
-      basic_auth_token = Base64.strict_encode64("#{@consumer_key}:#{@consumer_secret}")
+      basic_auth_token = encode_value("#{@consumer_key}:#{@consumer_secret}")
       "Basic #{basic_auth_token}"
     end
 
+    def encode_value(value)
+      [value].pack("m0").gsub("\n", '')
+    end
+
     def bearer_auth_header
       "Bearer #{@bearer_token}"
     end

spec/twitter/client_spec.rb

@@ -191,7 +191,7 @@
     it "creates the correct auth header with supplied consumer_key and consumer_secret" do
       uri = "/1.1/direct_messages.json"
       authorization = subject.send(:bearer_token_credentials_auth_header)
-      expect(authorization).to eq "Basic #{Base64.strict_encode64("CK:CS")}"
+      expect(authorization).to eq "Basic Q0s6Q1M="
     end
   end
 end