Implemented isIncomplete.

...cle/truffle/r/nodes/builtin/base/ConnectionFunctions.java

...om/oracle/truffle/r/runtime/conn/DelegateRConnection.java

.../truffle/r/runtime/conn/DelegateReadWriteRConnection.java

...acle/truffle/r/runtime/conn/DelegateWriteRConnection.java

...rc/com/oracle/truffle/r/runtime/conn/FileConnections.java

...me/src/com/oracle/truffle/r/runtime/conn/RConnection.java

...rc/com/oracle/truffle/r/runtime/conn/TextConnections.java

...le/truffle/r/test/builtins/TestBuiltin_rawConnection.java

...m/oracle/truffle/r/test/library/base/TestConnections.java