Don't modify the original PatternElements

Author: stasm

fluent.syntax/src/main/kotlin/org/projectfluent/syntax/processor/Processor.kt

@@ -37,7 +37,7 @@ class Processor {
             when (element) {
                 is TextElement -> {
                     if (lastText == null) {
-                        lastText = element
+                        lastText = TextElement(element.value)
                     } else {
                         lastText?.let { it.value += element.value }
                     }

fluent.syntax/src/test/kotlin/org/projectfluent/syntax/processor/ProcessorTest.kt

@@ -32,6 +32,26 @@ internal class ProcessorTest {
             processor.unescapeLiteralsToText(pattern)
         )
 
+        pattern.elements.clear()
+        pattern.elements.addAll(
+            arrayOf(
+                TextElement("Foo "),
+                Placeable(expression = StringLiteral("Bar"))
+            )
+        )
+        assertEquals(
+            Pattern(TextElement("Foo Bar")),
+            processor.unescapeLiteralsToText(pattern)
+        )
+        // The original Pattern isn't modified.
+        assertEquals(
+            Pattern(
+                TextElement("Foo "),
+                Placeable(expression = StringLiteral("Bar"))
+            ),
+            pattern
+        )
+
         pattern.elements.clear()
         pattern.elements.addAll(
             arrayOf(