Only cap the input if content length header is given

Currently RqLengthAware uses the InputStream#available to cap the input
if no Content-Length header is given, but the return value does not say
anything about the real content length, only how many bytes can be read
without blocking (what might be 0).

This now only returns a CapInputStream when a Content-Length is given in
the request and adds a testcase for the given case.

src/main/java/org/takes/rq/RqLengthAware.java

@@ -51,6 +51,10 @@
  */
 @EqualsAndHashCode(callSuper = true)
 public final class RqLengthAware extends RqWrap {
+    /**
+     * Constant for the Content-Length header.
+     */
+    private static final String CONTENT_LENGTH = "Content-Length";
 
     /**
      * Ctor.
@@ -68,14 +72,21 @@ public RqLengthAware(final Request req) {
      */
     private static InputStream cap(final Request req) throws IOException {
         final Iterator<String> hdr = new RqHeaders.Base(req)
-            .header("Content-Length").iterator();
-        InputStream body = req.body();
-        long length = (long) body.available();
+            .header(RqLengthAware.CONTENT_LENGTH).iterator();
+        final InputStream result;
         if (hdr.hasNext()) {
-            length = Long.parseLong(hdr.next());
+            final String value = hdr.next();
+            try {
+                result = new CapInputStream(req.body(), Long.parseLong(value));
+            } catch (final NumberFormatException ex) {
+                final String msg = "Invalid %s header: %s";
+                final String formated = String.format(msg, RqLengthAware.CONTENT_LENGTH, value);
+                throw new IOException(formated, ex);
+            }
+        } else {
+            result = req.body();
         }
-        body = new CapInputStream(body, length);
-        return body;
+        return result;
     }
 
 }

src/test/java/org/takes/rq/RqLengthAwareTest.java

@@ -23,6 +23,8 @@
  */
 package org.takes.rq;
 
+import java.io.ByteArrayInputStream;
+import java.io.FilterInputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.Arrays;
@@ -96,6 +98,32 @@ void readsByte() throws IOException {
         );
     }
 
+    @Test
+    void noContentLength() throws IOException {
+        final byte[] bytes = "test".getBytes();
+        final InputStream data = new FilterInputStream(new ByteArrayInputStream(bytes)) {
+            @Override
+            public int available() throws IOException {
+                return 1;
+            }
+        };
+        final InputStream stream = new RqLengthAware(
+            new RqFake(
+                Arrays.asList(
+                    "GET /test1",
+                    "Host: b.example.com"
+                ),
+                data
+            )
+        ).body();
+        for (final byte element : bytes) {
+            MatcherAssert.assertThat(
+                stream.read(),
+                Matchers.equalTo(element & 0xFF)
+            );
+        }
+    }
+
     @Test
     void readsByteArray() throws IOException {
         final String data = "array";