Replace backslashes in paths "correctly" on UNIX

The FirefoxOptions attempts to standardise paths to use
forward slashes since these are most portable. The
existing code works on Windows, but not a UNIX-like OS,
which is a pity.

The fix here will obviously fail if given a UNIX path
with a valid backslash in it, but we'll assume that
never happens.

java/client/src/org/openqa/selenium/firefox/FirefoxOptions.java

@@ -27,7 +27,6 @@
import static org.openqa.selenium.remote.CapabilityType.VERSION;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Joiner;
import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.gson.JsonArray;
@@ -197,12 +196,7 @@ public FirefoxOptions setBinary(Path path) {
  }

  private String toForwardSlashes(Path path) {
    Path root = path.getRoot();
    if (root != null) {
      return root.toString().replace('\\', '/') + Joiner.on("/").join(path);
    } else {
      return Joiner.on("/").join(path);
    }
    return Preconditions.checkNotNull(path).toString().replace('\\', '/');
  }

  public FirefoxOptions setBinary(String path) {