This worked for me:
$ phantomjs -v
1.9.2
$ gem search -l webdriver
*** LOCAL GEMS ***
selenium-webdriver (2.38.0)
watir-webdriver (0.6.4)
$ irb
irb(main):001:0> require "watir-webdriver"
=> true
irb(main):002:0> capabilities = Selenium::WebDriver::Remote::Capabilities.phantomjs
=> #<Selenium::WebDriver::Remote::Capabilities:0x007fbbeb8faa08 @capabilities={:browser_name=>"phantomjs", :version=>"", :platform=>:any, :javascript_enabled=>true, :css_selectors_enabled=>true, :takes_screenshot=>true, :native_events=>false, :rotatable=>false, :firefox_profile=>nil, :proxy=>nil}>
irb(main):003:0> capabilities['phantomjs.page.customHeaders.Accept-Language'] = 'ru-RU'
=> "ru-RU"
irb(main):004:0> browser = Watir::Browser.new :phantomjs, desired_capabilities: capabilities
=> #<Watir::Browser:0x5be4c6adc1d41736 url="about:blank" title="">
irb(main):005:0> browser.goto "httpbin.org/headers"
=> "http://httpbin.org/headers"
irb(main):006:0> puts browser.html
<html><head></head><body><pre style="word-wrap: break-word; white-space: pre-wrap;">{
"headers": {
"Accept-Encoding": "gzip",
"User-Agent": "Mozilla/5.0 (Macintosh; PPC Mac OS X) AppleWebKit/534.34 (KHTML, like Gecko) PhantomJS/1.9.2 Safari/534.34",
"Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8",
"Connection": "close",
"Accept-Language": "ru-RU",
"Host": "httpbin.org"
}
}</pre></body></html>
=> nil