Fix build on windows by using native file separator

On Windows platform the `ant` build was failing with ...

    [junit] Testsuite: fitnesse.responders.files.FileResponderTest
    [junit] Tests run: 15, Failures: 1, Errors: 0, Time elapsed: 0.977 sec
    [junit]
    [junit] Testcase: testFileContent took 0.067 sec
    [junit] Testcase: test304IfNotModifiedForClasspathResources took 0.055 sec
    [junit] Testcase: testRecoverFromUnparseableDateInIfNotModifiedHeader took 0.066 sec
    [junit] Testcase: test304IfNotModifiedForFiles took 0.126 sec
    [junit] Testcase: testLastModifiedHeaderForClasspathResources took 0.042 sec

    [junit] Testcase: shouldNotDealWithEscapedDirectoryOutsideFilesFolder took 0.06 sec
    [junit] Testcase: testLastModifiedHeader took 0.058 sec
    [junit] Testcase: shouldNotDealWithDirectoryOutsideFilesFolder took 0.061 sec
    [junit] Testcase: testClasspathResourceContent took 0.048 sec
    [junit] Testcase: shouldTellIfFileIsInFilesFolder took 0.05 sec
    [junit] Testcase: testNotFoundFile took 0.055 sec
    [junit] Testcase: testSpacesInFileName took 0.051 sec
    [junit]     FAILED
    [junit] expected:<.[/]TestDir\files\test F...> but was:<.[\]TestDir\files\test F...>
    [junit] junit.framework.AssertionFailedError: expected:<.[/]TestDir\files\test F...> but was:<.[\]TestDir\files\test F...>
    [junit]     at fitnesse.responders.files.FileResponderTest.testSpacesInFileName(FileResponderTest.java:83)
    [junit]
    [junit] Testcase: testUrlEncodedSpacesInFileName took 0.046 sec
    [junit] Testcase: testCssMimeType took 0.057 sec
    [junit] Testcase: testNavigationBackToFrontPage took 0.086 sec

    BUILD FAILED

This was caused by `FitNesseContext#getRootPagePath` hardcoding the path separator to /.