Http\Headers undefined index when calling ::fromString(' content-type: text/plain')

Hi,

This is my first contribution, so please try to be patient with me :)
When calling:

```
$headers = Zend\Http\Headers::fromString(' content-type: text/plain');
```

we get a

PHP Notice:  Undefined index: line in /home/markus/projects/zf2/library/Zend/Http/Headers.php on line 74
PHP Notice:  Undefined index: name in /home/markus/projects/zf2/library/Zend/Http/Headers.php on line 87


Would it be an option to throw an exception?

```
diff --git a/library/Zend/Http/Headers.php b/library/Zend/Http/Headers.php
index 6843fe1..b3ec64b 100644
--- a/library/Zend/Http/Headers.php
+++ b/library/Zend/Http/Headers.php
@@ -70,8 +70,16 @@ class Headers implements Countable, Iterator
                     'line' => trim($line)
                 );
             } elseif (preg_match('/^\s+.*$/', $line, $matches)) {
-                // continuation: append to current line
-                $current['line'] .= trim($line);
+                if ($current) {
+                    // continuation: append to current line
+                    $current['line'] .= trim($line);
+                } else {
+                    // Line does not match header format!
+                    throw new Exception\RuntimeException(sprintf(
+                        'Line "%s" does not match header format!',
+                        $line
+                    ));
+                }
             } elseif (preg_match('/^\s*$/', $line)) {
                 // empty line indicates end of headers
                 break;

``` 

Or maybe the first regex could match one or more whitespaces at the beginning

```
// check if a header name is present
if (preg_match('/^\s*(?P<name>[^()><@,;:\"\\/\[\]?=}{ \t]+):.*$/', $line, $matches)) {
```

Or is it totally okay to get this notices?