Fix multiline copyright header rule

Author: krwq

src/Microsoft.DotNet.CodeFormatting.Tests/Rules/CopyrightHeaderRuleTests.cs

@@ -29,6 +29,25 @@ class C
 
             var expected = @"// test
 
+class C
+{
+}";
+            Verify(source, expected);
+
+        }
+
+        [Fact]
+        public void CSharpSimpleMultiline()
+        {
+            _options.CopyrightHeader = ImmutableArray.Create("test1", "test2");
+            var source = @"
+class C
+{
+}";
+
+            var expected = @"// test1
+// test2
+
 class C
 {
 }";
@@ -48,6 +67,27 @@ class C
 
             var expected = @"// test
 
+class C
+{
+}";
+            Verify(source, expected);
+
+        }
+
+        [Fact]
+        public void CSharpPreserveExistingMultiline()
+        {
+            _options.CopyrightHeader = ImmutableArray.Create("test1", "test2");
+            var source = @"// test1
+// test2
+
+class C
+{
+}";
+
+            var expected = @"// test1
+// test2
+
 class C
 {
 }";

src/Microsoft.DotNet.CodeFormatting/Rules/CopyrightHeaderRule.cs

@@ -79,6 +79,7 @@ private List<string> GetExistingHeader(SyntaxTriviaList triviaList)
                 {
                     headerList.Add(GetCommentText(triviaList[i].ToFullString()));
                     i++;
+                    MovePastBlankLines(triviaList, ref i);
                 }
 
                 return headerList;