Fix handling of consecutive _send* calls

Previously, two consecutive calls of _send* functions will execute
the first one only. This commit fixes this problem.

Example reproducing the bug:

  spawn(...)
    .expect('prompt')
    .sendLine('response')
    .sendEof()
    .run(...);
Merge pull request #18 from strongloop/fix-send-eof-after-send-line

Fix handling of consecutive _send* calls