add xml:space="preserve" to text runs

In MS Word 2015 (and possibly others), leading and trailing spaces are
ignored in text runs. This means that calling TextRun with
leading/trailing space would result in a document that didn't include
those spaces. The fix here (per http://officeopenxml.com/WPtext.php)
is to include an extra attribute on the "w:t" element that forces word
to recognize those spaces.

ts/tests/docx/run/run-components/text.ts

@@ -0,0 +1,22 @@
+import { expect } from "chai";
+import { Text } from "../../../../docx/run/run-components/text";
+import { Formatter } from "../../../../export/formatter";
+
+describe("Text", () => {
+    describe("#constructor", () => {
+        it("creates an empty text run if no text is given", () => {
+            const t = new Text("");
+            const f = new Formatter().format(t);
+            expect(f).to.deep.equal({"w:t": [{_attr: {"xml:space": "preserve"}}]});
+        });
+
+        it("adds the passed in text to the component", () => {
+            const t = new Text(" this is\n text");
+            const f = new Formatter().format(t);
+            expect(f).to.deep.equal({"w:t": [
+                {_attr: {"xml:space": "preserve"}},
+                " this is\n text",
+            ]});
+        });
+    });
+});