Fix validation of 0/8 IPv4 addresses

0.0.0.0/8 IPv4 addresses are not illegal, just "reserved." This change
brings `InetAddressUtils` in line with the JDK's `InetAddress`
implementation, which accepts these addresses without issue, e.g.:

```java
    InetAddress.getByAddress(new byte[]{ 0, 10, 25, 68 });
    InetAddress.getByName("0.10.25.68");
```

Author: rschmitt

httpcore5/src/main/java/org/apache/hc/core5/net/InetAddressUtils.java

@@ -59,8 +59,7 @@ private InetAddressUtils() {
     }
 
     private static final String IPV4_BASIC_PATTERN_STRING =
-            "(([1-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\\.){1}" + // initial first field, 1-255
-            "(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\\.){2}" + // following 2 fields, 0-255 followed by .
+            "(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\\.){3}" + // First 3 fields, 0-255, followed by .
              "([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])"; // final field, 0-255
 
     private static final Pattern IPV4_PATTERN =

httpcore5/src/test/java/org/apache/hc/core5/net/TestInetAddressUtils.java

@@ -40,6 +40,7 @@ void testValidIPv4Address() {
         Assertions.assertTrue(InetAddressUtils.isIPv4("127.0.0.1"));
         Assertions.assertTrue(InetAddressUtils.isIPv4("192.168.0.0"));
         Assertions.assertTrue(InetAddressUtils.isIPv4("255.255.255.255"));
+        Assertions.assertTrue(InetAddressUtils.isIPv4("0.168.0.0"));
     }
 
     @Test
@@ -48,7 +49,6 @@ void testInvalidIPv4Address() {
         Assertions.assertFalse(InetAddressUtils.isIPv4("g.ar.ba.ge"));
         Assertions.assertFalse(InetAddressUtils.isIPv4("192.168.0"));
         Assertions.assertFalse(InetAddressUtils.isIPv4("256.255.255.255"));
-        Assertions.assertFalse(InetAddressUtils.isIPv4("0.168.0.0"));    //IP address that starts with zero not allowed
     }
 
     @Test