fix: IContainerGebConfiguration fileDetector (#151 / #146)

[JonasPammer]

#151 didnt include feature by #146, so these errors happened for me:

ChildPreferenceInheritedConfigSpec

Cannot invoke "geb.test.GebTestManager.getBrowser()" because the return value of "grails.plugin.geb.ContainerGebSpec.getTestManager()" is nulljava.lang.NullPointerException: Cannot invoke "geb.test.GebTestManager.getBrowser()" because the return value of "grails.plugin.geb.ContainerGebSpec.getTestManager()" is null
	at grails.plugin.geb.support.delegate.BrowserDelegate$Trait$Helper.getBrowser(BrowserDelegate.groovy:50)
	at grails.plugin.geb.support.delegate.BrowserDelegate$Trait$Helper.methodMissing(BrowserDelegate.groovy:408)
	at grails.plugin.geb.WebDriverContainerHolder$WebDriverContainerConfiguration.<init>(WebDriverContainerHolder.groovy:230)
	at grails.plugin.geb.WebDriverContainerHolder.reinitialize(WebDriverContainerHolder.groovy:91)
	at grails.plugin.geb.GrailsContainerGebExtension.visitSpec_closure2(GrailsContainerGebExtension.groovy:75)

InheritedConfigSpec + MultipleInheritanceSpec

No signature of method: geb.navigator.DefaultNavigator.fileDetector() is applicable for argument types: () values: []groovy.lang.MissingMethodException: No signature of method: geb.navigator.DefaultNavigator.fileDetector() is applicable for argument types: () values: []
	at geb.navigator.DefaultNavigator.methodMissing(DefaultNavigator.groovy:808)
	at geb.content.PageContentSupport.methodMissing(PageContentSupport.groovy:35)
	at geb.Page.methodMissing(Page.groovy:110)
	at grails.plugin.geb.support.delegate.BrowserDelegate$Trait$Helper.methodMissing(BrowserDelegate.groovy:408)
	at grails.plugin.geb.WebDriverContainerHolder$WebDriverContainerConfiguration.<init>(WebDriverContainerHolder.groovy:230)
	at grails.plugin.geb.WebDriverContainerHolder.reinitialize(WebDriverContainerHolder.groovy:91)
	at grails.plugin.geb.GrailsContainerGebExtension.visitSpec_closure2(GrailsContainerGebExtension.groovy:75)

[JonasPammer]

.\gradlew integrationTest --tests "org.demo.spock.ChildPreferenceInheritedConfigSpec" --tests "org.demo.spock.InheritedConfigSpec" --tests "org.demo.spock.MultipleInheritanceSpec" --stacktrace --info

[matrei]

Thank you!