Dependencies with transitive dependencies of type pom cause build exception

[tschulte]

When there is a dependency which has a transitive dependency of type pom, this causes a build exception. This can also be reproduced by depending on the pom directly.

When having following dependency

dependencies {
    jnlp 'log4j:log4j:1.2.17@pom'
}

this exception is raised

Caused by: java.util.zip.ZipException: error in opening zip file
at de.gliderpilot.gradle.jnlp.SignJarsTask.copyUnsignAndAlterManifest(SignJarsTask.groovy:53)
at de.gliderpilot.gradle.jnlp.SignJarsTask$_signJars_closure3_closure7.doCall(SignJarsTask.groovy:40)
at groovyx.gpars.pa.GParsPoolUtilHelper$_eachParallelPA_closure8.doCall(GParsPoolUtilHelper.groovy:192)
... 10 more