Fix URIBuilder encoding policy documentation and null reset

[prafgup]

URIBuilder defaults to ALL_RESERVED, but setEncodingPolicy's Javadoc incorrectly identifies RFC_3986 as the default. Passing null also selects RFC_3986 behavior instead of restoring the documented default.

This change corrects the Javadoc and makes null restore ALL_RESERVED.

Incorrect doc -

• httpcomponents-core/httpcore5/src/main/java/org/apache/hc/core5/net/URIBuilder.java

  Lines 206 to 216 in 4021b98

  	/**
  	* Sets the encoding policy for this {@link URIBuilder}.
  	* The encoding policy determines how URI components (e.g., query, fragment) are
  	* percent-encoded when building the URI string. If not set, the default policy
  	* is {@link EncodingPolicy#RFC_3986}.
  	*
  	* @param encodingPolicy the encoding policy to apply, or {@code null} to reset
  	* to the default ({@link EncodingPolicy#ALL_RESERVED})
  	* @return this {@link URIBuilder} instance for method chaining
  	* @since 5.4
  	*/

Current Default -

• httpcomponents-core/httpcore5/src/main/java/org/apache/hc/core5/net/URIBuilder.java

  Line 91 in 4021b98

  private EncodingPolicy encodingPolicy = EncodingPolicy.ALL_RESERVED;

Validation: ./mvnw -pl httpcore5 verify passed. All 1,380 tests passed, with no failures, errors, or skips.

[ok2c]

Cherry-picked to 5.4.x