TextIO.Write not handling URI properly

I think I have found a bug in TextIO, in the way it handles URIs. I am told that the TextIO.Write.to(String) method should take a URI string, but this doesn't seem to work for me.

Test case inline:

```

import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

import
java.io.File;
import java.io.IOException;
import java.net.URI;
import java.nio.file.*;
import java.util.*;
import
org.apache.beam.sdk.io.TextIO;
import org.apache.beam.sdk.testing.TestPipeline;
import org.apache.beam.sdk.transforms.Create;
import
org.apache.beam.sdk.values.PCollection;
import org.junit.Rule;
import org.junit.Test;

public class
Beam3429Test {

@Rule
public final transient TestPipeline pipeline = TestPipeline.create();

@Test
public void testBeamTextIO() throws IOException {
List words = Arrays.asList("tom",
"huck", "polly");
Create.Values source = Create.of(words);
PCollection
coll = this.pipeline.apply(source);
String fileName = "test" + System.currentTimeMillis() +
".txt";
String fileURI = new File(fileName).toPath().toUri().toString();
// ie file:///full/unix/path/to/test.file

coll.apply(TextIO.write().to(fileURI));
//test passes if we use the line below instead
of the above
// coll.apply(TextIO.write().to(fileName));
pipeline.run();

//read all sharded files:
Path dir = Paths.get(URI.create(fileURI)).getParent();

List files = new ArrayList<>();
Files.newDirectoryStream(dir, new DirectoryStream.Filter()
{
@Override public boolean accept(Path entry) throws IOException {
return
entry.toString().contains(fileName);
}
}).forEach(path -> files.add(path));

assertFalse("no files produced!", files.isEmpty());
List fileContents = new
ArrayList<>();
for (Path f : files) {
fileContents.addAll(Files.readAllLines(f));

}
assertTrue(fileContents.contains("tom"));
assertTrue(fileContents.contains("polly"));

}
}

```

Imported from Jira [BEAM-3429](https://issues.apache.org/jira/browse/BEAM-3429). Original Jira may contain additional context.
Reported by: mdarwin.

Start with the inline Beam3429Test reproduction and the TextIO.write().to(String) entry point. Run the test with a file URI and compare it with the plain-path case. Done means URI output creates the expected sharded files and the existing content assertions pass.

Written by the indexing model from the issue text.