fix(body-digest): fix the digest may be duplicated when it is at index 0

[tklai]

When the digest is at index 0, array_search() will return 0 but the if statement will become !0, which indicates the digest is not found in the $headerList and the program appends it to the array.

Before

[
  0 => "digest"
  1 => "(request-target)"
]

After

[
  0 => "digest"
  1 => "(request-target)"
  2 => "digest"
]

This change will improve the checking and only append digest when array_search() returns false.

EDIT: Added the missing 'if' before 'statement'