[435450] fixed by initializing context attribute in constructor of org.vertx.java.core.impl.BlockingAction.java

Signed-off-by: jerome Doucet <jerdct@gmail.com>
[435450] testOpenAndWriteFileSynch methods added for testing the fix in org.vertx.java.tests.core.filesystem.JavaFileSystemTest.java and vertx.tests.core.filesystem.TestClient

Signed-off-by: jerome Doucet <jerdct@gmail.com>
Merge pull request #881 from jeromedoucet/bug_435450

Bug 435450