Command task failing in Bamboo 5.1 if executable path is a shell command

Steps to reproduce

1. Create a command task with the path pointing to a shell command say echo for example. See the attached screenshot
2. When this is run against an older version of Bamboo, it runs fine despite the error message that "Unable to find executable at echo"

   error	03-Sep-2013 16:58:50	Unable to find executable at echo
   command	03-Sep-2013 16:58:50	Beginning to execute external process for build 'TESTA - TSTA - Default Job'\n ... running command line: \necho 2 > sultan.txt\n ... in: D:\atlassian\BAMBOO\atlassian-bamboo-4.4.4\HOME\xml-data\build-dir\TESTA-TSTA-JOB1\n
   

3. Now if this is done in the latest Bamboo 5.1, it fails with the build logs throwing something like

   error	03-Sep-2013 17:02:19	Unable to find executable at echo
   command	03-Sep-2013 17:02:19	Beginning to execute external process for build 'TESTA - TSTA - Default Job #17 (TESTA-TSTA-JOB1-17)'\n ... running command line: \necho 17 > sultan.txt\n ... in: D:\atlassian\BAMBOO\atlassian-bamboo-5.1.0\HOME\xml-data\build-dir\TESTA-TSTA-JOB1\n
   simple	03-Sep-2013 17:02:19	Failing task since return code of [echo 17 > sultan.txt] was -1 while expected 0
   

   The atlassian-bamboo.log shows something like:

   com.atlassian.utils.process.ProcessNotStartedException: echo could not be started
   	at com.atlassian.utils.process.ExternalProcessImpl.start(ExternalProcessImpl.java:288)
   	at com.atlassian.utils.process.ExternalProcessImpl.execute(ExternalProcessImpl.java:409)
   	at com.atlassian.bamboo.process.ProcessServiceImpl.executeExternalProcess(ProcessServiceImpl.java:211)
   	at com.atlassian.bamboo.plugins.shell.task.AbstractShellBuildTask.execute(AbstractShellBuildTask.java:61)
   	at com.atlassian.bamboo.plugins.shell.task.AbstractShellBuildTask.execute(AbstractShellBuildTask.java:48)
   	at com.atlassian.bamboo.task.TaskExecutorImpl.executeTasks(TaskExecutorImpl.java:220)
   	at com.atlassian.bamboo.task.TaskExecutorImpl.execute(TaskExecutorImpl.java:98)
   	at com.atlassian.bamboo.build.pipeline.tasks.ExecuteBuildTask.call(ExecuteBuildTask.java:75)
   	at com.atlassian.bamboo.v2.build.agent.DefaultBuildAgent.build(DefaultBuildAgent.java:189)
   	at com.atlassian.bamboo.v2.build.agent.BuildAgentControllerImpl.waitAndPerformBuild(BuildAgentControllerImpl.java:102)
   	at com.atlassian.bamboo.v2.build.agent.DefaultBuildAgent$1.run(DefaultBuildAgent.java:108)
   	at com.atlassian.bamboo.build.pipeline.concurrent.NamedThreadFactory$2.run(NamedThreadFactory.java:55)
   	at java.lang.Thread.run(Thread.java:724)