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

@@ -52,7 +52,9 @@
 @EqualsAndHashCode(callSuper = true)
 public final class RqLengthAware extends RqWrap {
 
-    /**
+    private static final String CONTENT_LENGTH = "Content-Length";
+
+	/**
      * Ctor.
      * @param req Original request
      */
@@ -68,14 +70,16 @@ 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();
-        if (hdr.hasNext()) {
-            length = Long.parseLong(hdr.next());
-        }
-        body = new CapInputStream(body, length);
-        return body;
+            .header(CONTENT_LENGTH).iterator();
+		if(hdr.hasNext()) {
+			String value = hdr.next();
+			try {
+				return new CapInputStream(req.body(), Long.parseLong(value));
+			} catch(NumberFormatException e) {
+				throw new IOException(String.format("Invalid %s header: %s", CONTENT_LENGTH, value));
+			}
+		}
+        return req.body();
     }
 
 }

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

@@ -23,9 +23,12 @@
  */
 package org.takes.rq;
 
+import java.io.ByteArrayInputStream;
+import java.io.FilterInputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.Arrays;
+
 import org.hamcrest.MatcherAssert;
 import org.hamcrest.Matchers;
 import org.junit.jupiter.api.Test;
@@ -95,6 +98,34 @@ void readsByte() throws IOException {
             Matchers.equalTo(data.length() - 1)
         );
     }
+
+    @Test
+    void noContentLength() throws IOException {
+        byte[] bytes = "test".getBytes();
+		final InputStream data = new FilterInputStream(new ByteArrayInputStream(bytes)) {
+        	@Override
+        	public int available() throws IOException {
+        		//This simulates a stream where only one byte is available at once
+        		return 1;
+        	}
+        };
+        final InputStream stream = new RqLengthAware(
+            new RqFake(
+                Arrays.asList(
+                    "GET /test1",
+                    "Host: b.example.com"
+                    //We have no content length header so RqLengthAware should simply do nothing
+                ),
+                data
+            )
+        ).body();
+        for(int i = 0; i < bytes.length; i++) {
+        	MatcherAssert.assertThat(
+        			stream.read(),
+        			Matchers.equalTo(bytes[i] & 0xFF)
+        			);
+		}
+    }
 
     @Test
     void readsByteArray() throws IOException {