Fix spec deprecations

This pull request fixes the deprecation warnings from running `bundle exec rake`. Rather than add an additional dependency for `rspec-its`, I rewrote 3 of the specs to have the same meaning without the `its` syntax.

```
Deprecation Warnings:

Use of rspec-core's `its` method is deprecated. Use the rspec-its gem instead. Called from /Users/jch/projects/ruby-net-ldap/spec/unit/ber/ber_spec.rb:123:in `block (3 levels) in <top (required)>'.
Use of rspec-core's `its` method is deprecated. Use the rspec-its gem instead. Called from /Users/jch/projects/ruby-net-ldap/spec/unit/ber/ber_spec.rb:130:in `block (3 levels) in <top (required)>'.
Use of rspec-core's `its` method is deprecated. Use the rspec-its gem instead. Called from /Users/jch/projects/ruby-net-ldap/spec/unit/ber/ber_spec.rb:137:in `block (3 levels) in <top (required)>'.

`be_false` is deprecated. Use `be_falsey` (for Ruby's conditional semantics) or `be false` (for exact `== false` equality) instead. Called from /Users/jch/projects/ruby-net-ldap/spec/unit/ldap/search_spec.rb:26:in `block (3 levels) in <top (required)>'.

`be_true` is deprecated. Use `be_truthy` (for Ruby's conditional semantics) or `be true` (for exact `== true` equality) instead. Called from /Users/jch/projects/ruby-net-ldap/spec/unit/ber/ber_spec.rb:123:in `block (4 levels) in <top (required)>'.
`be_true` is deprecated. Use `be_truthy` (for Ruby's conditional semantics) or `be true` (for exact `== true` equality) instead. Called from /Users/jch/projects/ruby-net-ldap/spec/unit/ber/ber_spec.rb:130:in `block (4 levels) in <top (required)>'.
`be_true` is deprecated. Use `be_truthy` (for Ruby's conditional semantics) or `be true` (for exact `== true` equality) instead. Called from /Users/jch/projects/ruby-net-ldap/spec/unit/ber/ber_spec.rb:137:in `block (4 levels) in <top (required)>'.
Too many uses of deprecated '`be_true`'. Pass `--deprecation-out` or set `config.deprecation_stream` to a file for full output.

`expect { }.not_to raise_error(SpecificErrorClass)` is deprecated. Use `expect { }.not_to raise_error` (with no args) instead. Called from /Users/jch/projects/ruby-net-ldap/spec/unit/ber/ber_spec.rb:94:in `block (3 levels) in <top (required)>'.
```
The gemspec locked it to rspec-expectations 2.99 for me (~> 2.0), which will give the following deprecation warning:

```
`expect { }.not_to raise_error(SpecificErrorClass)` is deprecated. Use `expect { }.not_to raise_error` (with no args) instead. Called from /Users/jch/projects/ruby-net-ldap/spec/unit/ber/ber_spec.rb:94:in `block (3 levels) in <top (required)>'.
```

The positive matcher (expect raises) can take an exception, but the negative matcher (expect not raises) doesn't. https://www.relishapp.com/rspec/rspec-expectations/v/2-99/docs/built-in-matchers/raise-error-matcher#expect-no-error-at-all