set_client_encoding: Use error message provided by libpq

... instead of our own, less specific text.

This is a better fix for #267 : https://bitbucket.org/ged/ruby-pg/issues/267

ext/pg_connection.c

@@ -2978,7 +2978,7 @@ pgconn_set_client_encoding(VALUE self, VALUE str)
 	Check_Type(str, T_STRING);
 
 	if ( (gvl_PQsetClientEncoding(conn, StringValueCStr(str))) == -1 ) {
-		rb_raise(rb_ePGerror, "Failed to set client encoding (invalid encoding name: %s or connection error)", StringValueCStr(str));
+		rb_raise(rb_ePGerror, "%s", PQerrorMessage(conn));
 	}
 #ifdef M17N_SUPPORTED
 	pgconn_set_internal_encoding_index( self );

spec/pg/connection_spec.rb

@@ -1253,6 +1253,12 @@
 				expect( escaped.encoding ).to eq( Encoding::ISO8859_1 )
 				expect( escaped.encode ).to eq( "\"Möhre to\"".encode(Encoding::ISO8859_1) )
 			end
+
+			it "raises appropriate error if set_client_encoding is called with invalid arguments" do
+				expect { @conn.set_client_encoding( "invalid" ) }.to raise_error(PG::Error, /invalid value/)
+				expect { @conn.set_client_encoding( :invalid ) }.to raise_error(TypeError)
+				expect { @conn.set_client_encoding( nil ) }.to raise_error(TypeError)
+			end
 		end
 
 		describe "respect and convert character encoding of input strings" do