GpxFile.BuildString omits duplicate namespaces.

Resolves #33

Author: airbreather

doc/release-notes.md

@@ -3,6 +3,7 @@
 ## [0.5.0](https://github.com/NetTopologySuite/NetTopologySuite.IO.GPX/milestone/10)
 - `GpxWriterSettings.TimeZoneInfo` is no longer completely ignored ([#30](https://github.com/NetTopologySuite/NetTopologySuite.IO.GPX/issues/30)).
 - Fix an issue where written timestamps were being rounded to the nearest second instead of preserving as many significant digits as `DateTime` allows ([#31](https://github.com/NetTopologySuite/NetTopologySuite.IO.GPX/issues/31)).
+- Tweak the settings used for `GpxFile.BuildString` to avoid repeating the `xmlns` declaration for extensions that are declared in the standard GPX namespace ([#33](https://github.com/NetTopologySuite/NetTopologySuite.IO.GPX/issues/33)).
 
 ## [0.4.0](https://github.com/NetTopologySuite/NetTopologySuite.IO.GPX/milestone/7)
 - `GpxReaderSettings` now has an `IgnoreBadDateTime` property to use for ignoring a timestamp value that we cannot parse as an instance of the `DateTime` struct, to work around a `0000-00-00T00:00:00Z` coming from CompeGPS ([#29](https://github.com/NetTopologySuite/NetTopologySuite.IO.GPX/issues/29)).

src/NetTopologySuite.IO.GPX/GpxFile.cs

@@ -26,6 +26,7 @@ public sealed class GpxFile
             Indent = true,
             NewLineOnAttributes = true,
             OmitXmlDeclaration = true,
+            NamespaceHandling = NamespaceHandling.OmitDuplicates,
         };
 
         private GpxMetadata metadata = new GpxMetadata(creator: "NetTopologySuite.IO.GPX");

tests/NetTopologySuite.IO.GPX.Tests/GpxFileTests.cs

@@ -2,6 +2,7 @@
 using System.IO;
 using System.Linq;
 using System.Text;
+using System.Text.RegularExpressions;
 using System.Xml;
 using System.Xml.Linq;
 
@@ -348,5 +349,22 @@ public void TimestampsShouldPreserveFractionalSecondsWithinDefinedPrecision()
             Assert.Contains("5432-10-10T11:22:33.8765432Z", text); // DateTime resolution is 100ns, so the value gets rounded to 7 digits
             Assert.Contains("1111-11-11T11:11:11.12345Z", text); // don't output extra zeroes
         }
+
+        [Fact]
+        [Regression]
+        [GitHubIssue(33)]
+        public void ExtensionsElementInGpxNamespaceShouldNotRepeatNamespaceSpecByDefault()
+        {
+            const string GpxText = @"
+<gpx xmlns='http://www.topografix.com/GPX/1/1' version='1.1' creator='airbreather'>
+    <extensions><myTest someData='4' /></extensions>
+</gpx>
+";
+            string text = GpxFile.Parse(GpxText, null).BuildString(null);
+
+            // be careful when using regex to match XML.
+            var namespaceWasRepeatedRegex = new Regex("<extensions>.*topografix.*</extensions>", RegexOptions.Singleline);
+            Assert.DoesNotMatch(namespaceWasRepeatedRegex, text);
+        }
     }
 }