Fix StringStreamDataWriter.toString to not consume buffer (#384)

During debugging, it was confusing to see some contents of the buffer
when pausing in the debugger before an assertion, but then seeing the
assertion fail. It didn't make sense why the assertion fails - the value
of the buffer was as expected.

It turned out that the debugger called `toString()`, consuming the
buffer, and leaving an empty buffer when performing the assertion. The
solution here is to just `peek()` the buffer in `toString()`, not
consume it.

src/commonMain/kotlin/it/krzeminski/snakeyaml/engine/kmp/api/StringStreamDataWriter.kt

@@ -16,5 +16,8 @@ internal class StringStreamDataWriter(
         buffer.writeUtf8(string = str, beginIndex = off, endIndex = off + len)
     }
 
-    override fun toString(): String = buffer.readUtf8()
+    /**
+     * Returns the contents of the internal buffer, without altering the buffer's state.
+     */
+    override fun toString(): String = buffer.peek().readUtf8()
 }

src/commonTest/kotlin/it/krzeminski/snakeyaml/engine/kmp/api/StringStreamDataWriterTest.kt

@@ -0,0 +1,19 @@
+package it.krzeminski.snakeyaml.engine.kmp.api
+
+import io.kotest.assertions.withClue
+import io.kotest.core.spec.style.FunSpec
+import io.kotest.matchers.string.shouldContain
+
+class StringStreamDataWriterTest: FunSpec({
+    test("toString() doesn't consume buffer") {
+        val writer = StringStreamDataWriter()
+        writer.write("foobar")
+
+        // First read.
+        writer.toString()
+
+        withClue("subsequent calls to toString() shouldn't change the buffer's state") {
+            writer.toString() shouldContain "foobar"
+        }
+    }
+})