Seeking before beginning of file should fail

When seeking in a file to a location before the beginning of the file,
the operation seems to succeed. While if i try to do the same with a
"real" file then the seek command will return an error.

So i changed the seek method to return false if the offset is less
than 0. This seems to solve my problem and behaves in the same
way as seeking in a real file. However I do not know if there are
situations where seeking before the beginning of the file could be
a valid use case. testing with fseek on unix fs always returns -1
when seeking past the beginning of the file. Added five test
assertions to validate this behavior.

src/main/php/org/bovigo/vfs/content/SeekableFileContent.php

@@ -52,24 +52,30 @@ protected abstract function doRead($offset, $count);
      */
     public function seek($offset, $whence)
     {
+        $old_offset = $this->offset;
         switch ($whence) {
             case SEEK_CUR:
                 $this->offset += $offset;
-                return true;
+                break;
 
             case SEEK_END:
                 $this->offset = $this->size() + $offset;
-                return true;
+                break;
 
             case SEEK_SET:
                 $this->offset = $offset;
-                return true;
+                break;
 
             default:
                 return false;
         }
+
+        if ($this->offset<0) {
+            $this->offset = $old_offset;
+            return false;
+        }
 
-        return false;
+        return true;
     }
 
     /**

src/test/php/org/bovigo/vfs/vfsStreamFileTestCase.php

@@ -129,6 +129,8 @@ public function seekEmptyFile()
         $this->assertEquals(0, $this->file->getBytesRead());
         $this->assertTrue($this->file->seek(2, SEEK_END));
         $this->assertEquals(2, $this->file->getBytesRead());
+        $this->assertFalse($this->file->seek(-1, SEEK_SET),'Seek before beginning of file');
+        $this->assertEquals(2, $this->file->getBytesRead());
     }
 
     /**
@@ -158,6 +160,9 @@ public function seekRead()
         $this->assertTrue($this->file->seek(2, SEEK_END));
         $this->assertEquals('', $this->file->readUntilEnd());
         $this->assertEquals(11, $this->file->getBytesRead());
+        $this->assertFalse($this->file->seek(-35, SEEK_SET),'Seek before beginning of file');
+        $this->assertEquals('', $this->file->readUntilEnd());
+        $this->assertEquals(11, $this->file->getBytesRead());
     }
 
     /**