Ensure credentials set in Twitter.configure are of a valid type.

This commit adds basic credential validation after the credentials
are set using the Twitter.configure block. If the credentials are
not a string or symbol, a Twitter::Error::ConfigurationError is
raised with a helpful message.

lib/twitter/configurable.rb

@@ -1,3 +1,5 @@
+require 'twitter/error/configuration_error'
+
 module Twitter
   module Configurable
     attr_writer :consumer_key, :consumer_secret, :oauth_token, :oauth_token_secret
@@ -21,8 +23,12 @@ def keys
     end
 
     # Convenience method to allow configuration options to be set in a block
+    #
+    # @raise [Twitter::Error::ConfigurationError] Error is raised when supplied
+    #   twitter credentials are not a String or Symbol.
     def configure
       yield self
+      validate_credential_type!
       self
     end
 
@@ -61,5 +67,20 @@ def options
       Hash[Twitter::Configurable.keys.map{|key| [key, instance_variable_get(:"@#{key}")]}]
     end
 
+    # Ensures that all credentials set during configuration are of a
+    # valid type. Valid types are String and Symbol.
+    #
+    # @raise [Twitter::Error::ConfigurationError] Error is raised when
+    #   supplied twitter credentials are not a String or Symbol.
+    def validate_credential_type!
+      credentials.each do |credential, value|
+        next if value.nil?
+
+        unless value.is_a?(String) || value.is_a?(Symbol)
+          raise(Error::ConfigurationError, "Invalid #{credential} specified: #{value} must be a string.")
+        end
+      end
+    end
+
   end
 end

lib/twitter/error/configuration_error.rb

@@ -0,0 +1,8 @@
+require 'twitter/error'
+
+module Twitter
+  class Error
+    class ConfigurationError < ::ArgumentError
+    end
+  end
+end

spec/twitter_spec.rb

@@ -64,6 +64,30 @@
         expect(Twitter.instance_variable_get(:"@#{key}")).to eq key
       end
     end
+
+    context "when invalid credentials are provided" do
+      it "raises a ConfigurationError exception" do
+        expect {
+          Twitter.configure do |config|
+            config.consumer_key = [12345, 54321]
+            config.consumer_secret = 'valid_data'
+          end
+        }.to raise_exception(Twitter::Error::ConfigurationError)
+      end
+    end
+
+    context "when no credentials are provided" do
+      it "does not raise an exception" do
+        expect {
+          Twitter.configure do |config|
+            config.consumer_key = nil
+            config.consumer_secret = nil
+            config.oauth_token = nil
+            config.oauth_token_secret = nil
+          end
+        }.to_not raise_exception(Twitter::Error::ConfigurationError)
+      end
+    end
   end
 
   describe ".credentials?" do